Deep understanding of PVC plasticity
1. Effect of plasticization on product performance
The performance of PVC products is closely related to the degree of plasticization. The plasticization degree is not good, the products are brittle, and the mechanical properties cannot meet the requirements.
The plasticizing degree is too high, the products will appear yellow line, the mechanical properties are not qualified, and the plasticizing degree is in PVC products. It is very important during the processing.
In the past, we have experience, saying that the performance is best when the plasticizing degree is 60%~70%, and the plasticizing degree of the material for producing the water supply pipe is 60%-65%. Because it can reflect two properties of tensile strength and impact strength in this range. However, is the PVC Dg 60%~70%, the performance of the product is the best?
Mr. Wang Wenzhi, senior engineer of PVC products industry, issued a paper at Baotou “Hot Stabilizer Environmental Seminar” in June 2017: ISO 1452-2:2009 with longitudinal tensile strength ≥45MPa, elongation at break ≥80%, Tp≥ Three indicators, such as 185 ° C, are used to evaluate the degree of plasticization. All three indicators are ≥, indicating that the greater the plasticity requirement, the better. The impact strength peaked with an increase in Dg, and other properties were monotonically elevated.
Summers et al. believe that the higher the processing temperature, the higher the strength of the three-dimensional network structure formed by PVC gelation, the lower the creep of the product, and the higher the tear strength and the long-term stress resistance. As for the increase in processing temperature, the toughness of the product peaks, not because the Dg rises, but the impact strength decreases, but the melt fracture and the slip agent failure occur due to the increase of the processing temperature.
2. Effect of temperature and shear on plasticization
The plasticization of PVC resin is a process in which particles move, melt, and adhere to each other to form a melt under the combined action of mechanical force and heat. Therefore, mechanical forces (shear, compression, etc.) and heat have a direct effect on the plasticization of PVC resin.
The PVC resin can be plasticized by heating alone at a sufficiently high temperature. Photograph of PVC resin with heat stabilizer added at 220 ° C, after 1 minute and 3 minutes:
The state after 3 minutes indicates that the PVC molecular chain is opened, the fluidity is strong, and it attracts and fuses with each other. However, the plasticization performed in this state has no practical significance. First, the thermal stability is limited, and second, the performance after plasticization does not meet the requirements.
Since the individual heating does not meet the requirements, the combination of mechanical shearing and heating will bring more realistic results. The viscosity of the PVC melt is large, and therefore, in the usual processing temperature range, the resin is difficult to plasticize by heating alone. According to the plasticizing behavior mode and microscopic mechanism of PVC resin, if the material is sheared and compressed while heating, it will promote the close contact of the PVC primary particles with each other, thereby promoting plasticization.
Under normal processing conditions, the plasticizing rate of PVC resin increases with increasing processing temperatures and shear rates. As the processing temperature increases, the greater the temperature difference, the faster the heat transfer rate. Since PVC is a poor conductor of heat, the increase of shear speed will accelerate the frictional heat generation between materials, and at the same time accelerate the contact frequency between materials and equipment, and improve the heat exchange efficiency.
In the absence of plasticizer, PVC material can not form a melt at low temperature, in a glass state, the material in glass state is hard and brittle, the material can not be processed in the glass state; as the temperature rises to 160 ° C The material is in a high elastic state, but the material still cannot flow in this area, which can only make the material soft and viscoelastic; if it can achieve PVC melt processing and has fluidity, the temperature should be between 160-200 °C. .
However, for any stabilizer, when the temperature is higher than 200 ° C, the material is decomposed for a long time, so when controlling the degree of plasticization, the temperature can only be controlled between 160-200 ° C. In the temperature range of 40 ° C, the plasticization is better when the temperature of the PVC is set between 170 and 180 ° C.
3. Effect of resin structure on plasticization degree
Different processing techniques result in differences in the degree of porosity of the PVC resin particles. The loose resin has a faster melting rate than the compact resin. For example, the loose bulk resin is melted faster than the compact bulk method.
The glass transition temperature and melting point of PVC increase with the increase of molecular weight and crystallinity, and the plasticization of PVC also becomes difficult.
4, the effect of PVC formula on plasticization
The use of lubricants, plasticizers, processing aids, impact modifiers, fillers, stabilizers, etc. in the processing of PVC has a great influence on the properties of PVC plasticization. Of course, the different components have different effects on the plasticizing properties of PVC due to their different purposes.
As a simple example, when we do a rheological test, it is easy to find that as the number of CPE increases, the plasticizing time is shortened, indicating that CPE can promote plasticization. As the stabilizer increases, the plasticizing time is prolonged, and since it contains external lubrication, the amount of the stabilizer is increased, and the amount of the external lubricant naturally increases. All additives such as ACR and slip agent will have an effect on plasticizing behavior.
5. Effect of mixing and processing on plasticization
Mixing is a process in which a PVC resin is mixed with a heat stabilizer, a modifier, a lubricant, a filler colorant, and the like. The equipment used is mainly a high speed kneader and a cooling mixer.
The mixing process relies on the frictional force and shearing force generated by the mechanical force on the material to make the material refine and heat up, so that some additives are melted and coated on the surface of the PVC resin. The PVC resin is refined under the action of shearing and friction. Under the action of temperature, the surface is soft and porous. The auxiliary agent is adsorbed on the surface and homogenized. The temperature is further increased, the surface of the particle is melted, and the density of the particles is increased.
In this process, under the strong stirring of PVC resin particles, when the material temperature is below 50 °C, the agglomerated powder particles and larger particles are rubbed, and the granulated particles are pulverized and become smaller. At this time, the apparent density of the dry mixture is increased. The change is small and the resin particles become small. When the temperature of the material is above 80 ° C to 120 ° C, the PVC resin particles expand larger (the coefficient of thermal expansion near the glass transition temperature is larger), the particle size tends to be uniform, and the average size of the particles is similar to that of the unmixed PVC resin particles. At the same time, the apparent density of the dry blend increases rapidly as the particles absorb the auxiliaries.
The process is not complicated, but the quality of the mix directly affects the quality of normal production and products. The uniformity of the mixing, as well as the uniformity of shear extrusion and heat transfer in the subsequent production process, can cause plasticization unevenness.
6. A simple method to increase the degree of plasticization during processing
1). By increasing the temperature of the fuselage and screw.
2). When the screw rotation speed is normal, increase the feed speed of the feeder to increase the plasticization degree.
3). Increase the speed of the extruder at the rated speed of the extruder and in the case of feeding.
4). Give the dry powder a good ripening period (12-48h).
5). Adjust the degree of plasticization by lowering or raising the temperature of the combined core.